Design of a semiconductor structure for light emitting diodes influences on overall efficiency of the diodes through two main parameters: efficiency of conversion of electrical power to optical power in light generation region and efficiency of emission of light generated in this region from the structure. In a light emitting structure made of nitrides of group III metals grown in vapor phase on foreign substrate with refractive index less than that of the structure materials, a significant part of generated light propagates inside the structure due to reflection on structure/substrate and structure/ambient interfaces. Only a part of light propagating within a certain critical angle, defined by Snell's law and related to the surface normal direction, leave the structure through the structure surface. This critical angle depends on refractive indices of structure material and substrate and ambient materials. Due to a significant difference in these refractive indices between the substrate (e.g., ≈1.8 for sapphire) and the ambient (e.g., ≈1.5 for typical encapsulating resins), as compared to the structure materials (≈2.5-3), this critical angle is relatively small. Up to two thirds of light can propagate in structure layers, serving as waveguide. In light emitting diode chips, this light is potentially capable to exit via chip sides. However, because of the many loss mechanisms present in the structure layers and electrodes, most of this light is lost before exiting chip sides. Because of this, the efficiency of light emission from the structure is significantly decreased, resulting in decreased overall device efficiency.
A number of methods including ex-situ technological operations constitute a significant part of the patents devoted to the problem. One method for improving capability of light to be emitted from the structure is to shape light emitting surface into a hemisphere. This method is disclosed by Scifres and Burnham in U.S. Pat. No. 3,954,534 and includes formation of hemispherical depressions on a substrate with subsequent growth of semiconductor layers over the substrate and removing the substrate away. Another solution is disclosed by Krames and Kish Jr. in U.S. Pat. No. 5,779,924. It is suggested to increase transmission of total optical power from the structure to ambient by fabricating ordered interface texturing. This texturing reduces Fresnel reflection at the interface between the structure and ambient and increase the critical angle, propagating within which light can leave the structure through the surface. Kish Jr. and Stockman in U.S. Pat. No. 5,793,062 suggested inserting non-absorbing distributed Bragg reflectors inside the structure designed to redirect light away from absorbing regions such as contacts within he chip. In fact, it is hard to grow laterally patterned distributed Bragg reflectors in case of nitrides of group III metals. Gardner et al in U.S. Pat. No. 6,847,057 B1 disclose a light emitting device, in which improved light diffusion is provided by texturing of the substrate surface, or structure surface, or one of internal structure interfaces. The invention also suggests using an optional polarization selection layer that polarizes the photons emitted by the active region. The polarization selection layer can be wire grid polarizer and may be formed on a side of the substrate opposite the device layers. A wire grid polarizer reflects photons of a polarization that is parallel to the wires, and transmits photons of a polarization that is perpendicular to the wires. The combination of the wire grid polarizer and reflecting texturing surface should recycle photons until they achieve a certain polarization. As already mentioned, one common disadvantage of these methods is that, although they can provide effective light diffusion, they require a number of ex-situ operations resulting in complicated manufacturing process.
Several in situ methods have also been suggested. Krames et al. in U.S. Pat. No. 6,649,440 B1 disclose an in situ method of making a light emitting device with improved light extraction efficiency. The method utilizes a thick multi-layered epitaxial structure that increases the light extraction efficiency from the device. The multi-layered structure does not absorb the light, and its increased thickness allows light trapped within a waveguide to escape the light emitting device through the sides of the structure with fewer reflections, thus avoiding light losses in active region and electrodes. A disadvantage of the method is that the multi-layered structure must be much thicker than light emitting region to provide significant improvement of light extraction from the device, thus resulting in significantly higher growth time and cost of such structure, as compared to conventional structures. Besides, thick multi-layered structure can induce significant strain in the light emitting structure. Krames et al. in U.S. Pat. No. 6,683,327 B2 disclose a light emitting device including a nucleation layer containing aluminum. The thickness and aluminum composition of the nucleation layer are selected to match the index of refraction of the substrate and device layers, such that 90% of light from the device layers incident on the nucleation layer is extracted into the substrate. One disadvantage of this method is that it is hard to grow in vapor phase light emitting structures above such a nucleation layer, having thickness required to provide effective light diffusion, without deteriorating structural quality of above grown layers. Thibeault et al in U.S. Pat. No. 6,821,804 B2 disclose several solutions based on creation of arrays of light extraction elements formed either within the structure or on the substrate prior to epitaxial growth. The array of light extraction elements are formed to provide a spatially varying index of refraction, so that light trapped within a waveguide interacts with the arrays, changes direction of propagation and can escape the light emitting device. These solutions improve significantly capability of light to be emitted from the structure; however inclusions of foreign materials can introduce additional defects in structure layers. Another proposed solution is insertion of disperser layers formed either within the structure or on the substrate prior to epitaxial growth. However to provide refractive index difference large enough for effective light refraction, a layer made of nitrides of group III metals should have thickness and composition, which can introduce significant additional strains in the structure. Shen et al in U.S. Pat. No. 6,903,376 B2 disclose a light emitting device, which includes light emitting region and a reflective contact separated from the light emitting region by one or more layers. The separation between the light emitting region and the reflective contact is between 0.5λn and 0.9λn or between λn and 1.4λn, etc, where λn is the wavelength of light emitted from the light emitting region in an area of the device separating the light emitting region and the reflective contact. According to the invention, light extraction efficiency of top-side flux as a function of the separation distance has maximums at certain values, because of the phase shift of light reflected from the reflective contact and interference of light directly emitted from the light emitting region and reflected from the contact. In fact, however, this phenomenon is efficient for thin single quantum well regions, but less pronounced in case of complex light emitting regions having several quantum wells. One common disadvantage of all the described in situ methods is that they result in additional strains in the structure with consequent increase of defect density.
As one of the latest solutions, Lee et al in US patent application 2005/0082546 A1 disclose a method, which includes formation of a substrate having at least one protruded portion with a curved surface, in which uniform stress distribution can be obtained. The device provides improved light extraction, while saving consistent defect density in the structure. One disadvantage of this method is that, although it provides effective light diffusion, it requires complicated manufacturing process, including ex-situ operations.